Gehinderte Ligandbewegungen in Übergangsmetallkomplexen, XXXVIII [1]. [4+5]-Cycloaddition von 2,3-Dimethyl-1,3-butadien an Tricarbonyl- η5-2,4-pentadienyl-mangan und Tricarbonyl-η5-dimethyl-2,4-pentadienyl-mangan / Hindered Ligand Motions in Transition Metal Complexes, XXXVIII [1]. [4+5] Cycloaddition of 2,3-Dimethyl-1,3-butadiene to Tricarbonyl(η5 -2 ,4 -pentadienyl)manganese and Tricarbonyl(η5 -2 ,4 -dimethyl-2 ,4 -pentadienyl)manganese

Abstract

Tricarbonyl(η5-2,4-pentadienyl)manganese (1) and tricarbonyl(η5-2,4-dimethyl-2,4-pentadienyl)manganese (2) photochemically add 2,3-dimethyl-1,3-butadiene (3). Mediated by manganese, 6,7-dimethyl-2,6-cyclononadiene-1-yl and 1,3,6,7-tetramethyl-2,6-cyclononadiene-1-yl complexes are formed. Η-shifts from the methyl groups of the substituted CC double bonds convert the primary products into tricarbonyl-η3:CH-l-methylene-2-methyl-6-cyclononen-2-ylmanganese (4), and tricarbonyl-η3:CH-1-methylene-2,5,7-trimethyl-6-cyclononen-2-yl-manganese (6), respectively. In addition tetracarbonyl-η3-1-methylene-2-methyl-6-cyclononen-2-yl-manganese (5), and tetracarbonyl-η3-1-methylene-2,5,7-trimethyl-6-cyclononen-2-ylmanganese (7) are obtained. The latter are also formed from 4 and 6 by addition of CO under ambient conditions. In the complexes 4 and 6 the hydrocarbon ligands are coordinated by enyl units, encompassing C 1, C2, and the exocyclic methylene group, and by a C-H-Mn 2-electron-3-center bond of the 2-methyl group to the manganese. They exhibit temperature dependent 1H NMR spectra due to a hindered rotation of the 2-methyl group with a barrier of activation of AG* = 40-43 kJ ·mol -1. There is evidence for an equilibrium of the complexes 4 and 6 with the isomers tricarbonyl-η3:CH-1-methylene-2-methyl-5-cyclononen-2-yl-manganese (4′), and tricarbonyl-η3:CH-1-methylene-2,5,7-trimethyl-5-cyclononen-2-yl-manganese (6′). At higher temperatures the isomers 4, 4′ and 6, 6′ are interconverted by a 1,4-H shift with a barrier of activation of AG* = 75 kJ · mol-1. The crystal and molecular structure of 4 was determined by X-ray structure analysis. The constitutions of the complexes 4-7 were deduced from IR and 1H NM R spectra.